1. Field of Invention
The present invention pertains to the field of exposure control in cameras. More particularly, this invention relates to exposure control in a digital camera.
2. Art Background
Cameras including digital still cameras and video cameras typically include an image sensor and an exposure mechanism for controlling the amount of light from an image scene that reaches the image sensor. Digital cameras, for example, usually include a charge-coupled device (CCD) array as an image sensor along with mechanisms that control aperture and shutter speed exposure settings.
Image scenes often include objects which are very bright in comparison to other objects. For example, light sources such as light bulbs, candles, etc., in a relatively dark image scene are usually very bright in comparison to the remainder of the image scene. In addition, direct reflections of light sources from highly reflective surfaces in an image scene are usually very bright in comparison to the remainder of the image scene.
Exposure control in a camera typically involves determining the exposure settings that best utilize the dynamic range of the image sensor given the characteristics of an image scene being photographed. The dynamic range of an image sensor may be defined as the ratio of its highest possible signal output to its lowest useful signal output. In a CCD array, for example, the highest possible signal output is usually limited by its charge capacity and its lowest useful signal output is usually limited by circuit noise.
Prior methods for exposure control in a camera typically involve measuring the average intensity of light from a sample area of an image scene being photographed and then selecting exposure settings that scale that average to a predetermined percentage of the dynamic range of the image sensor. However, light sources and reflections of light sources in the sample area usually yield a relatively high average intensity. A relatively high average intensity typically results in relatively low exposure settings using prior methods. Such low exposure settings typically reduce the intensity of light obtained from areas of the image scene outside of the light sources and reflections of light sources to the low end of the dynamic range of the image sensor. Unfortunately, this typically causes areas of interest in the image scene to appear too dark once photographed.